Fiber optic cables are used in electric utility substations and generating plants to carry digital data between pieces of equipment. Since the fiber optic cables are electrically insulating, their use eliminates the pick up of electrical noise signals from the switching of electric power circuits and from the very high point to point voltages created in a substation or generating plant by an electrical ground fault or lightning strike within the facility.
Fiber optic cables are also used in computer networks, climate control systems to carry digital signals from point to point within a building, and in extended manufacturing or military facilities, again to give freedom from induced electrical noise and protection from lightning strikes. Fiber optic cables are used to carry digital control signals in the Boeing 777 aircraft for the same reasons as above. Use of such fiber optic cables creates problems, however, not found in using metallic conductors for signal transmission, that are common regardless of where the fiber optic cables are used.
Fiber optic cables are usually terminated using a two part coupler, one fastened to the end of the cable and the other mounted on a piece of equipment. When mated, the two parts of the coupler position the end of a fiber to accept light from a light sender or to detect light coming from the end of a fiber by a light receiver. Most optical fiber lines carry light in only one direction and two lines are commonly joined side by side for two directional data transfer. Depending on the diameter of the glass or plastic fiber within the line, the couplers may require considerable precision and are quite expensive. Preparing an optic fiber line in the field and attaching one portion of the coupler can be especially difficult if glass fibers are used, requiring light measurements to assure good light transference.
Moreover, couplers for glass fiber lines differ from those for plastic lines. Devices which are to be used with glass fibers lines for long distance data transmission must, therefore have a different coupler than when used with plastic fiber lines for short distance data transmission. This difference results in added cost of manufacture, stocking, sales, and use of the device. Moreover, if the use of digital communications is optional, the manufacturer of the device must either install transmitting and receiving couplers and associated circuitry which may never be used for some customers of the device or manufacture two form of the device, one with and one without the provision for fiber optic communications.
Another problem for a manufacturer is that some customers, not wishing to use fiber optic cables, may require one of several types of electrical outputs for wire transmission of digital data.
Most microprocessors operate with a power supply of either +5 volts dc or +3 volts dc. Digital data is most readily available from devices using such microprocessors in signals going from zero volts to either +3 or +5 volts. From early use of circuitry having individual transistors, the term `transistor/transistor logic` (TTL) is commonly used to describe such circuits or signals.